With the increase of recording density of magnetic recording, the surface property of a magnetic layer becomes more smooth, so that there occurs a problem that the coefficient of friction of the magnetic layer with each part in a recording and reproducing apparatus is increased to hinder smooth running of the magnetic recording medium and to damage the magnetic layer.
For solving the aforesaid problem, a method of adding a fatty acid ester into a magnetic layer [as described in JP-A-50-22603, JP-A-50-153905, and JP-A-55-139637 (the term "JP-A" as herein used means an "unexamined published Japanese patent application") and JP-B-39-28367, JP-B-41-18065, and JP-A-47-12950 (the term "JP-B" as used herein means an "examined published Japanese patent application")], a method of adding a silicone compound into a magnetic layer (as described in U.S. Pat. No. 2,654,681), and a method of adding a fatty acid or a hydrocarbon into a magnetic layer are proposed and attempted.
Furthermore, with the popularization of video tape recorders (VTR), personal computers, word processors, etc., the conditions, in particular, the temperature and humidity ranges in use of magnetic recording media become wider recently. Also, magnetic recording media are used in the system of repeating the on-off action of a magnetic head onto the magnetic layer in the form of rotary media such as floppy disks for personal computers and word processors, and the running durability of the magnetic recording medium under a high temperature condition or a thermocycle of from a high temperature to a low temperature for a long time becomes an important issue. However, in such conventional techniques, there are problems that the aforesaid additives are liable to vaporize from the surface of the magnetic layer at a high temperature and on the other hand, the additives deposit on the surface of the magnetic layer at a low temperature to cause clogging of a magnetic head or increasing of drop out etc. Also, when a large amount of the additive is added to the magnetic layer for obtaining the increased effect, the additive plasticizes a binder resin of the magnetic layer to deteriorate the film strength of the magnetic layer, whereby the durability thereof is reduced.
Also, as another strategy for improving the running durability of a magnetic recording medium, a method of adding abrasives (particles having high hardness) into the magnetic layer thereof is proposed and practiced. However, in order to obtain the satisfactory effect, a considerably large amount of the abrasive must be added into the magnetic layer, which results in problems such as deterioration of the electromagnetic characteristics and increase of the abrasion of a magnetic head.
For solving the aforesaid problems, a method of adding a fatty acid ester having a high molecular weight and having a branched hydrocarbon group or an unsaturated hydrocarbon group to a magnetic layer so that the additive does not deposit on the surface of the magnetic layer at a low temperature is disclosed in JP-A-58-160425, JP-A-58-218038, JP-A-60-205827, JP 61-294637, and JP-A-62-125529.
However, since these esters are in a liquid state at normal temperature, are easily compatible with the binder resin, and plasticize the binder resin, film strength of the magnetic layer is reduced and the sufficient effect is not obtained. On the other hand, the ester compounds having a straight chain alkyl group are relatively excellent in lubricating property but many of them have a high melting point, and hence the use of such ester compounds frequently causes a problem that they tend to deposit on the surface of the magnetic layer at a low temperature.
Also, as other methods for solving the aforesaid problems, a method of using a fatty acid ester having a low melting point and a fatty acid ester having a high melting point in combination is disclosed in JP-A-61-294637 and a method of using fluid paraffin and a phthalic acid ester together is disclosed in JP-B-60-49972. However, these methods are yet insufficient for solving the aforesaid problems. A magnetic recording medium having a magnetic thin layer of a ferromagnetic metal (or alloy) also suffers from the aforesaid problems, and various techniques to solve the problems are disclosed in JP-A-63-83909, JP-A-63-83921, JP-A-63-83910, and JP-A-63-234225. However, these techniques can not sufficiently improve the running durability under the recent wide environmental conditions.
On the other hand, for meeting the requirement on high density magnetic recording, it is effective to sufficiently disperse a ferromagnetic powder in a binder resin and as a useful method, a method of using a polymer having a polar functional group such as a carboxy group, a sulfonic acid group, a phosphoric acid group, etc., introduced into the molecule is disclosed in JP-A-55-44227, JP-A-57-133521, JP-A-60-238309, and JP-A-61-104329.
The aforesaid method can improve the quality of the magnetic layer of floppy disks, etc., but at the same time the improvement of the dispersibility of a ferromagnetic powder increases the smoothness of the magnetic layer so that the aforesaid problem of deteriorated running durability becomes acute and hence the advantage of the technique is not sufficiently enjoyed.
As a binder resin for magnetic recording media, there are thermoplastic resins such as a vinyl chloride-vinyl acetate copolymer, and a vinyl chloride-vinylidene chloride resin, a cellulose series resin, an acetal series resin, a urethane series resin, etc. and they are used solely or as a combination thereof. However, in the case of using these thermoplastic resins, the abrasion resistance of the magnetic layer is insufficient and can not meet the aforesaid requirement of a magnetic recording medium having an excellent running durability. On the other hand, a method of using a thermosetting resin such as a melamine resin, a urea resin, etc., or a method of adding a compound having a crosslinking property by chemical reaction, such as an isocyanate compound or a compound having an epoxy ring to the aforesaid thermoplastic resin is proposed, thereby a magnetic layer having an excellent abrasion resistance can be obtained and the requirement for the improvement of the running durability is considerably fulfilled.
However, when the aforesaid crosslinking binder is used, there occur the following problems. That is, firstly, the dispersed system of a ferromagnetic powder is poor in storage stability, thereby the coating composition for a magnetic layer can not keep the uniformity of the property and thus the magnetic recording medium obtained using the coating composition can not keep the homogeneity in the characteristics thereof. Secondly, after coating the coating composition and drying, a heat treatment step for hardening the coated magnetic layer is inevitable and also a long time is required for the hardening step.
As a means free of such problems, a method of using acrylic acid ester series oligomer and monomer and, after coating and drying, curing the layer by irradiation of radiations is disclosed in JP-B-47-12423, JP-A-47-13639, JP-A-47-150104, JP-A-50-77455, and JP-A-56-25231. By the method, magnetic recording media having good abrasion resistance is obtained without causing the aforesaid problem in production step. However, in the method, magnetic recording media having both good electromagnetic characteristics and excellent running durability suitable for high density recording are not obtained.